The formation of clots within the circulatory system has been known to result in serious, if not fatal, consequences, particularly when the clot lodges within the heart or the brain. To prevent clots from occurring, coagulation inhibiting agents are normally administered to a patient. As a prerequisite for administering the coagulation inhibiting agents, the level of inhibiting agents within the patient's circulatory system must be known. The time required for clot formation within a standard laboratory test tube has been interpreted as an indication of the level of such inhibiting agents within a patient's circulatory system.
A system and a method for automatically measuring clotting time is disclosed in U.S. Pat. No. 3,695,842 entitled "METHOD AND SYSTEM FOR ANALYZING A LIQUID", issued to Michael D. Mintz on Oct. 30, 1972 and assigned to International Technidyne Corporation, the assignee of the present invention. A sample of blood is placed in a test tube and a permanent magnet is immersed in the blood sample. A magnetic reed switch, which is normally open, is positioned directly below the magnet. Flux lines, provided by the magnet, pass through the reed switch, causing it to close. Then, a relatively rotational motion is produced between the test tube and the magnet to agitate the blood, during which time the magnet remains positioned over the reed switch. When the blood coagulates, the resulting fibrous strands of clotted sample causes the magnet to move conjointly with the test tube. Thus, the magnet is displaced from the reed switch. This displacement causes a reduction in the density of the magnetic flux lines passing through the reed switch (i.e. weakens the magnetic field). As a result, the reed switch opens and a signal is generated, indicating the occurrence of the coagulation of blood.
An improved system for measuring clotting time is disclosed in U.S. Pat. No. 3,836,333 entitled "SYSTEM FOR TIMING THE COAGULATION OF BLOOD" issued to Michael D. Mintz on Oct. 30, 1972 and assigned to International Technidyne Corporation. An electromagnetic bias coil, which is wound around the reed switch, provides steady-state magnetic flux lines that supplement the flux lines provided by the permanent magnet. When the density of the flux lines passing through the reed switch decreases as a result of the magnet being displaced, the reed switch opens. The bias coil also provides a magnetic pulse, which forces the reed switch to a closed state. This system is manufactured under the trademark HEMOCHRON by International Technidyne Corporation at Edison, N.J.
The precision with which the system detects the coagulation of blood is dependant upon the ability of the reed switch to respond to changes in density of the magnetic flux lines. To ensure that the reed switch opens and closes in the presence of the correct flux densities, the reed switch must be manufactured with a great deal of precision. When tested, any reed switch that does not operate as specified must be eliminated. This results in additional expense as high precision switches are inherently more costly.
One problem encountered in using a reed switch for magnetic field measurement relates to the process of magnetic hysteresis. The effect of hysteresis in a reed switch is to require a greater magnetic flux density to initially close the reeds than that required to simply maintain the reeds in a closed condition. In the system described above, the difference between the magnetic field required to close the reeds and the magnetic field at which the reeds just reopened must be less than the difference in magnetic field passing through the reed switch when the magnet has been displaced relative to the reed switch.
A second problem with reed switches relates to magnetic storage or magnetization. The reeds of the switch are made of a ferromagnetic material. During operation, when magnetic flux lines pass through the reed switch, the reeds store energy at a slow rate and therefore, become magnetized. As a result of the stored magnetic energy, the reed switch becomes a time-dependent storage device, which may fail to open or close precisely at predetermined external flux level. Thus, a reed switch employed in the system described above may not switch states precisely when the blood clots.
Still a further improvement for detecting the coagulation of blood is disclosed in U.S. Pat. No. 5,154,082 entitled "MICROPROCESSOR-CONTROLLED APPARATUS AND METHOD FOR DETECTING THE COAGULATION OF BLOOD" issued to Michael D. Mintz on Oct. 13, 1992 and assigned to International Technidyne Corporation. In this patent a microprocessor calibrates the reed switch by adjusting the density of the magnetic flux lines from an adjustable source so that the reed switch is open when the ferromagnetic member reaches a predetermined distance of displacement. However, problems also exist with this improved device as well, since the biased reed switch acting as a clot detector timer is unable to determine the exact position of the magnet at a given point in time. The reed relay design only detects the presence of the calibrated field strength within the test well and not its position. Knowledge of the exact position of the magnet within the test tube would permit the actual clot time to be empirically extrapolated, as well as actually detected. A still further problem associated with the prior art analog base designs is the need to precisely calibrate using an empty test well each of the systems in order to accurately measure and detect the blood clotting. Moreover, because of the analog nature of the prior design of the prior art, the use of the reed relay and field coil bias apparatus causes signal drift and may result in miscalibration of the analog test well. Still further, the prior art uses, in addition to reed relay switches and wire coils, thermistors as the heating mechanism and temperature sensors. Each of these analog components are both difficult to manufacture and costly. Accordingly, a test well which can accurately track magnetic position, rate of magnet movement, ratio of movement from and to multiple positions, as well as provide detailed viscometry changes and accurate clotting information while eliminating signal and field strength drift, in addition to easing construction and lowering the cost, is highly desirable.